For mobile gas heaters, as used e.g. in camper vans or leisure craft, gas valves are used with which the gas flow to a gas burner of the gas heater is controlled. In simplified terms, the gas valve consists of a valve body in which a through-flow cross-section and a valve seat are provided. A valve element is provided which, starting from a closed position in which it lies against the valve seat and closes the through-flow cross-section, can be moved into an opened position in which the through-flow cross-section is cleared.
The valve element is connected to an armature to which a coil is associated. When the coil is excited, i.e. is supplied with current, it displaces the armature so that the valve element is moved into the opened position. Furthermore, a spring is provided which acts upon the armature or the valve element in such a way that the valve element is in the closed position when the coil is unpowered.
It is known from the prior art that two windings are allocated to the armature, namely a pulling winding and a holding winding.
The pulling winding serves to exert a large force on the armature and to displace it against the effect of the spring in such a way that the valve element moves into the opened position. A relatively high level of electric current is required for this purpose.
The holding winding serves to hold the armature in the opened position after it has been displaced by the pulling winding into this position. The force required to hold the armature in the opened position is smaller than the force which must be provided by means of the pulling winding. Consequently, the holding winding must also be supplied only with a comparatively low level of current which is in the order of 20% of the current of the pulling winding. The holding winding alone is not able to move the armature into the opened position.
When the gas valve is to be opened, a short current pulse is applied to the pulling winding in order to pull the armature into the opened position. Then, the holding winding is excited with a constant current in order to hold the armature in the opened position over a prolonged period.
A disadvantage of this is that, owing to the current pulse with which the pulling winding is excited in order to open the gas valve, the armature and also the valve element are displaced into the opened position at a high speed. In so doing, they strike against a stop, whereby noise and mechanical vibrations are produced throughout the system.
DE 102 16 484 A1 also discloses exciting a solenoid of an electromagnetic valve with a pulse width-modulated current. The pulse duty factor is reduced at a predetermined time so that the speed of displacement of the armature is reduced. Nevertheless, it is not possible to reliably prevent the armature from coming into contact with the stop at an undesirably high speed.
The object of the invention is to reduce the noise produced during opening of the gas valve.